This invention relates to storage batteries of the lead-acid type. These batteries, commonly used as standby power sources, industrial traction and stationary batteries, and in automotive and other ignition systems, contain at least one cell consisting of positive electrodes of lead peroxide and negative electrodes of spongy lead, which are immersed in a sulfuric acid electrolyte.
Because lead-acid storage battery cells are of the secondary type, the battery can be recharged at any point during the discharge portion of its charge/discharge cycle by proper application of an external current source, such as a voltage-regulated rectifier charger, alternator or generator which passes current through the battery cells in a direction opposite to that in which the cells supply current to a load. The float voltage of the battery thus can be continuously maintained during operation of an associated system so that ideally it will deliver approximately its full current capacity when it is employed to supply power to that system.
Lead-acid storage batteries are customarily stored, before installation by the ultimate user, in either a "wet charged" or "dry charged" condition. Wet charged batteries are initially charged during manufacture and are shipped to the user with the electrolyte in contact with the electrode plates in the cell compartments. Dry charged batteries are supplied with dry but active plates and are also charged at the source of manufacture, but are stored and shipped to the user in a dry condition to be activated at the time of installation by filling the cells with an acid electrolyte.
Particularly in recent years, dry charged batteries have become an important segment of the lead-acid storage battery market. This may be largely because dry charged batteries have at least partially overcome some of the problems associated with the storage of fully-charged wet batteries including, for example, handling a battery filled with acid electrolyte, gradual loss of capacity during storage necessitating periodic recharging, and severe element corrosion and sulfation within battery cells which may limit the service life of the battery. However, problems have been encountered with dry charged batteries in general, for example, loss of capacity between manufacture and first discharge due to element corrosion.
In 1906, a process of treating the iron-nickel alkaline storage batteries common at that time was disclosed by Thomas A. Edison in U.S. Pat. No. 817,162. Such batteries are described in this patent as having lost capacity during shipment, following removal of the electrolyte. Edison reported that the loss of capacity was caused by exothermic oxidation from air of iron of the negative electrode, which could be prevented by sending a reversing current through the battery to discharge the negative electrode completely.
Many modern lead-acid batteries that have been dry charged have been found to be susceptible to failure in service due to premature corrosion of the positive elements. Conventional manufacturing practices usually produce charged and dried batteries having positive plates comprising a significant amount of bare metal, produced during the operations of brushing, cutting and burning necessary to assemble the cell. It is well known that corrosion of the positive elements can be controlled by mild and continuous anodization of this bare metal when the battery is in service. However, a process is needed which, after the addition of acid electrolyte to the battery, will provide initial anodization of the bare metal of the positive elements and thus prevent catastrophic corrosion and consequent loss of electrical capacity.
It is now believed that a reason for premature positive grid corrosion and subsequent failure of dry charged storage batteries made pursuant to conventional methods is a local cell on individual plates of the positive grid of the battery, consisting essentially of a positive of the active electrode material and a negative of the bare metal of the grid. Unless the electrical potential of the positive plate is maintained at a significantly higher level than its normal rest potential, a spontaneous local discharge reaction occurs, and may continue until failure of the plate and ultimately of the battery.
In order to circumvent the problem of positive grid corrosion in activated dry charged storage batteries, the conventional recommended practice is to boost-charge the batteries after addition of the electrolyte. However, the recommended procedure, to boost-charge all cells at a voltage of 2.6 to 2.7 volts per cell, sometimes is not followed. This may be because the voltage necessary to accomplish such boost-charging is not provided by a charger available where the battery is activated, or because the directions supplied with the battery are in a language which is foreign to the user and not easily understood. In fact, even the necessity for the initial boost-charge may not be grasped by the user.
It has also been conventional practice in the art of manufacturing dry charged batteries to control the parameters of the process used to charge and dry the negative plates to provide a maximum charge thereon. Metallic lead--the principal constitutent of a negative electrode--has a nautral tendency to oxidize in moist air, forming lead oxide; consequently, great care has always been taken to prevent this natural process from occurring in order to maintain the negative plate in a condition of maximum charge.